1. Field of the Invention
The present invention relates to an AM receiver circuit, and more particularly to an AM receiver circuit which performs sound quality compensation in accordance with field intensity of a received broadcast wave when the field intensity is low.
2. Description of the Related Art
According to the amplitude modulation (AM) method, a signal to be transmitted (modulation signal) is transmitted on the amplitude of a carrier wave having a frequency that can be broadcast from a broadcast station. The AM method is mainly employed for medium frequency radio broadcast (526.5 to 1606.5 kHz). A radio wave within the medium-length broadcast frequency band is characterized in that it propagates not only as a ground wave, but also as a space wave, especially during nighttime, which is reflected by the ionosphere (E layer) located approximately 100 km from the ground. Accordingly, use of a medium-length radio wave makes it possible to serve a large service area. Further it is also possible to provide a stable service to moving bodies such as vehicles.
A receiver for receiving an AM signal typically employs the super-heterodyne detection method. According to the super-heterodyne detection method, a signal from a broadcast station is synthesized with a signal generated by an oscillator (local oscillator) circuit included within the receiver so as to be converted into an intermediate frequency wave, and the converted signal is subsequently amplified and demodulated. This method is advantageous in that high amplification gain can be easily obtained and crosstalk can be minimized. In order to select a desired broadcast wave, a band-pass filter that permits passage of the frequency of that particular broadcast wave alone is required. Because it is very difficult to continuously change the center frequency of a band-pass filter without changing the band characteristic of the filter, the local oscillation frequency is typically altered to convert received signals into intermediate frequency waves having a predetermined frequency, such that waves having only the predetermined frequency need be passed.
In an output obtained at a speaker of an AM receiver, the amount of noise generated by an amplifier or the like increases in relation to the signal as the field intensity of the broadcast wave input into the antenna becomes lower.
In order to maintain an output audio signal at a constant level even when the field intensity of the broadcast wave input into the antenna fluctuates, a receiver for receiving an AM signal is generally provided with an AGC (automatic gain control) circuit for adjusting the amplification factor of an RF signal amplifier or intermediate frequency amplifier.
A technique of changing, in accordance with the field intensity of a received broadcast wave, the sensitivity of an AGC circuit for amplifying a received RF signal is known, as disclosed in Japanese Patent Laid-Open Publication No. Hei 7-22975, for example.
Further, in order to minimize auditory unpleasantness caused by a decrease in the signal-to-noise (s/n) ratio during input of a low field intensity signal, a sound quality compensation circuit for compensating sound quality is often provided in an audio signal circuit disposed in a stage subsequent to a detecting section.
For example, in an FM receiver, a technique of controlling a tone control circuit in accordance with the field intensity of a received broadcast wave so as to adjust the sound quality of an output signal is known, as disclosed in Japanese Patent Laid-Open Publication No. 2000-13340.
In a receiver for receiving an AM signal, an AGC control voltage (hereinafter referred to as the signal strength (S) meter signal output) of an AGC circuit is conventionally employed as information regarding the field intensity of a received broadcast wave signal.
A conventional sound quality compensation method used in an AM receiver circuit is described as follows. FIG. 1 is a schematic diagram showing a configuration of an example conventional AM receiver circuit 100. Broadcast wave signals received by an antenna 112 are input into a front end (FE) section 114. The FE section 114 includes an RF amplifier and a tuning circuit, and serves to amplify and selectively tune to a broadcast wave signal for supplying an output. An intermediate frequency (IF) signal section 116 serves to convert the frequency of a carrier wave. The IF signal section 116 comprises a local oscillator for outputting a signal having a frequency that differs by a predetermined level (typically 450 kH) from that of a received broadcast wave, and a mixer for mixing the broadcast wave signal and the local oscillator signal. Using those components, the IF signal section 116 converts the carrier wave frequency of a desired broadcast wave selected from among the received broadcast waves into a predetermined intermediate frequency (typically 450 kH). Further, using a band-pass filter (BPF) having the center frequency corresponding to the intermediate frequency, the desired signal alone is extracted and amplified for output as the intermediate frequency (IF) signal. The AM receiver circuit 100 further comprises an IF-AGC circuit 118 for maintaining constant intensity of the IF signal output from the IF signal section 116. The IF-AGC circuit 118 receives a portion of the output from the IF signal section 116, generates an AGC control voltage (signal strength meter signal), and supplies the voltage as feedback to the IF signal section 116. In this manner, the IF-AGC circuit 118 controls the gain of the amplifier of the IF signal section 116.
The signal output from the IF signal section 116 is input into an AM detection section 120. The AM detection section 120 removes the carrier wave component from the IF signal to obtain an audio signal, which is the original modulation signal. A sound quality compensation section 122 employs the S meter signal (signal strength meter signal) generated by the IF-AGC circuit 118 as a signal reflecting information regarding the received broadcast field intensity, and performs sound quality compensation in accordance with the S meter signal.
FIG. 2 is a graph showing a relationship between the intensity (denoted by a solid line) of a signal input into the amplifier of the IF signal section 116 and the degree of amplification (denoted by a dashed line) performed by the amplifier, given as functions of the field intensity received at the antenna 112. Generally speaking, when the received field intensity becomes higher, the intensity of signal input into the amplifier increases while the degree of amplification is reduced by the function of the AGC circuit 118. However, because there is a maximum limit to the degree of amplification that can be performed by the amplifier of the IF signal section 116 (amplification factor of the circuit), the degree of amplification remains at a constant level regardless of the received field intensity when the received field intensity is below E1.
FIG. 3 illustrates a relationship between the degree of amplification (denoted by a dashed line) performed by the amplifier of the IF signal section 116 and the S meter signal output (denoted by a solid line), given as functions of the field intensity received at the antenna 112. When the received field intensity is at the level of E1 or higher, the AGC circuit 118 outputs the S meter signal in accordance with the field intensity of the received broadcast wave signal, and the degree of amplification by the amplifier is controlled by the AGC circuit 118 so as to supply an output signal having a constant intensity. However, when the received field intensity is below E1, the S meter signal denoting the detection level is not output because the S meter signal sensitivity is low. Accordingly, at a received field intensity below E1, the AGC circuit 118 fails to function, resulting in a constant degree of amplification by the amplifier of the IF section.
As described above, in a conventional method using the S meter signal output as the information denoting the field intensity of the received broadcast wave, sound quality compensation can be performed in accordance with the field intensity only when the field intensity is greater than E1. When the field intensity of the received broadcast wave is below E1 (under a low field intensity environment), no field intensity information can be obtained, resulting in a failure to perform appropriate sound quality compensation.